LED-Filaments and LED-Filament Lamps

ABSTRACT

An LED-filament includes a partially light-transmissive substrate; blue LED chips mounted on a front face of the substrate; first broad-band green to red photoluminescence materials and a first narrow-band manganese-activated fluoride red photoluminescence material covering the blue LED chips and the front face of the substrate; and second broad-band green to red photoluminescence materials covering the back face of the substrate. The LED-filament may further include a second narrow-band manganese-activated fluoride red photoluminescence material on the back face of the substrate in an amount that is less than 5 wt % of a total red photoluminescence material content on the back face of the substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/540,019, filed Aug. 13, 2019, which in turn claims the benefit ofpriority to U.S. provisional application Ser. No. 62/831,699, filed Apr.9, 2019, each of which is hereby incorporated by reference in itsentirety.

FIELD OF THE INVENTION

Embodiments of the present invention are directed to LED-filaments andLED-filament lamps. More particularly, although not exclusively, theinvention concerns LED-filaments and LED-filament lamps that generatelight having a General Color Rendering Index CRI Ra of at least 80.

BACKGROUND OF THE INVENTION

White light emitting LEDs (“white LEDs”) include one or morephotoluminescence materials (typically inorganic phosphor materials),which absorb a portion of the blue light emitted by the LED and re-emitlight of a different color (wavelength). The portion of the blue lightgenerated by the LED that is not absorbed by the phosphor materialcombined with the light emitted by the phosphor provides light whichappears to the eye as being white in color. Due to their long operatinglife expectancy (>50,000 hours) and high luminous efficacy (100 lm/W andhigher), white LEDs are rapidly being used to replace conventionalfluorescent, compact fluorescent and incandescent lamps.

Recently, LED-filament lamps have been developed comprisingLED-filaments whose visual appearance resemble the filament of atraditional incandescent lamp. The LED-filaments, which are typically 2inches (52 mm) long, comprise COG (Chip-On-Glass) devices having aplurality of low-power LED chips mounted on one face of alight-transmissive glass substrate. Front and back faces of thelight-transmissive substrate are coated with a phosphor-impregnatedencapsulant, such as silicone. Typically, the phosphor comprises amixture of green and red light emitting phosphors for generating warmwhite light and to increase General Color Rendering Index (CRI Ra) oflight generated by the filament. The same phosphor-impregnatedencapsulant is applied to both faces of the substrate to ensure that thefilament generates the same color of light in forward and backwarddirections.

Narrow-band red phosphors such as, for example, manganese-activatedfluoride phosphors such as K₂SiF₆:Mn⁴⁺(KSF), K₂TiF₆:Mn⁴⁺(KTF), andK₂GeF₆:Mn⁴⁺(KGF) have a very narrow red spectrum (Full Width HalfMaximum of less than 10 nm for their main emission line spectrum) whichmakes them highly desirable for attaining high brightness (about 25%brighter than broad-band red phosphors such as europium-activated rednitride phosphor materials such as CASN—CaAlSiN₃:Eu) and high CRI Ra ingeneral lighting applications. While manganese-activated fluoridephotoluminescence materials are highly desirable, there are drawbacksthat make their use in LED-filaments challenging. For example, theabsorption capability of manganese-activated fluoride phosphors issubstantially lower (typically about a tenth) than that ofeuropium-activated red nitride phosphor materials are currently used inLED-filaments. Therefore, in order to achieve the same target colorpoint, the usage amount of manganese-activated fluoride phosphorstypically can be from 5 to 20 times greater than the usage amount of acorresponding europium-activated red nitride phosphor. The increasedamount of phosphor usage significantly increases the cost of manufacturesince manganese-activated fluoride phosphors are significantly moreexpensive than europium-activated red nitride phosphors (at least fivetimes more expensive). Moreover, compared with packaged LEDs, sinceequal amounts of phosphor are required on each side of the filament,this doubles usage amount of manganese-activated fluoridephotoluminescence material. As a result of the higher usage and highercost, use of narrow-band manganese-activated fluoride red phosphors isprohibitively expensive for LED-filaments.

Embodiments of the invention concern improvements relating to theLED-filaments and LED-filament lamps and in particular, although notexclusively, reducing the cost of manufacture of LED-filaments withoutcompromising on brightness and CRI Ra through innovative phosphorpackaging structures to improve the blue absorption efficiency ofmanganese-activated fluoride photoluminescence material.

SUMMARY OF THE INVENTION

Some embodiments of the invention concern LED-filaments that areconfigured to generate a majority (e.g. at least 70% of the total) oflight in a forward direction away from a front face of the substrate onwhich the LED chips are mounted and a small proportion of light in abackward direction away from the back face of the substrate. Moreparticularly, the substrate and LED chips are configured such that theproportion of total blue excitation light generated by the blue LEDchips on (emanates from) the front face side of the substrate issubstantially greater (e.g. at least 70% of the total) than on (emanatesfrom) the opposite back face side. Such a configuration enables use of ahigher brightness narrow-band red phosphor on the front face of thesubstrate only and a less expensive red phosphor other than amanganese-activated fluoride phosphor (“non-manganese-activated fluoridephotoluminescence material” also referred to as a broad-band redphotoluminescence material) on the back face of the substrate whilestill providing substantially most of the superior brightness benefit ofusing narrow-band manganese activated fluoride on both faces, but usingonly half (50% by weight) the quantity of narrow-band redphotoluminescence material. This is to be contrasted with knownLED-filaments which use the same photoluminescence materials on thefront and back faces of the substrate to ensure a uniform color emissionin forward and backward directions. In accordance with the invention,the LED-filament can be configured in the above way by, for example,using: (i) a partially light-transmissive substrate, (ii) LED chips thatgenerate more light from a top face in a forward/upward direction thanin a backward/downward direction from a bottom face (base) towards thesubstrate, (iii) providing a reflector, or partial reflector, on thebase of one or more of the LED chips or a combination thereof. Thepresent invention finds particular utility in LED-filaments that use anat least partially light-transmissive substrate.

In some embodiments, an LED-filament comprises a partiallylight-transmissive substrate having a plurality of blue LED chipsmounted on a front face of the substrate; narrow-band red and firstbroad-band green to red photoluminescence materials disposed on andcovering the front face of the substrate and the plurality of blue LEDchips; and second broad-band green to red photoluminescence materialscovering the opposite back face of the substrate, there being only asmall quantity or no narrow-band photoluminescence material present onthe back face. The narrow-band and broad-band red photoluminescencematerials typically have different crystal structures—that is the redphotoluminescence material covering the front face has a differentcrystal structure to that of the red photoluminescence material coveringthe back face. In an embodiment, the narrow-band red photoluminescencematerial comprises a manganese-activated fluoride photoluminescencematerial (e.g. KSF), and the broad-band red photoluminescence materialcomprises rare-earth activated red photoluminescence material, forexample, CASN. In this patent specification “broad-band redphotoluminescence material” and “non manganese-activated fluoridephotoluminescence material” denotes a red photoluminescence materialwhose crystal structure is other than that of a manganese-activatedfluoride red photoluminescence material, such as for examplerare-earth-activated red photoluminescence materials including forexample a red emitting nitride-based phosphor, a Group selenide sulfideor silicate-based photoluminescence (phosphor) material.

According to an embodiment, an LED-filament comprises: a partiallylight-transmissive substrate; a plurality of blue LED chips mounted on afront face of the substrate; first broad-band green to redphotoluminescence materials and a first narrow-band manganese-activatedfluoride red photoluminescence material covering the plurality of blueLED chips and the front face of the substrate; and second broad-bandgreen to red photoluminescence materials covering the back face of thesubstrate. The inventors have discovered that by providing thenarrow-band manganese-activated fluoride red photoluminescence on onlythe front face of the substrate and a less expensive second broad-bandphotoluminescence material on the back face of the substrate providessubstantially the same brightness increase benefit but uses only half(50% by weight) the quantity of manganese-activated fluoridephotoluminescence material. In embodiments, the LED-filament can furthercomprises a second narrow-band manganese-activated fluoride redphotoluminescence material on the back face of the substrate in anamount that is less than 5 wt % of a total red photoluminescencematerial content on the back face of the substrate. Embodiments of theinvention comprise 0 wt % of the second narrow-band manganese-activatedfluoride red photoluminescence material on the back face of thesubstrate.

In embodiments, the substrate has a transmittance of at least one of: 2%to 70%, 30% to 50% and 10% to 30%. In embodiments, at least one of: atleast 70%, at least 80%, and at least 90% of the total blue lightgenerated by the LED chips is on the front face side of the substrate.Since, in embodiments of the invention, the substrate is only partiallylight-transmissive and/or the LED chips have a reflector covering theirbase, a greater proportion of the total blue excitation light generatedby the blue LED chips will be on (emanates from) the front face side ofthe substrate than on the back face side of the substrate. It will beappreciated that this is true even when the LED chips generate equalamounts of blue excitation light in forward (i.e. away from the frontface of the substrate) and backward (i.e. towards the front facesubstrate) directions since the substrate will allow passage of only aproportion of blue excitation light to pass and reflect the remainderresulting in greater proportion of blue excitation light on the frontface side of the substrate. Due to this difference in the proportion oftotal blue excitation light on opposite faces of the substrate, itenables use of a less expensive broad-band red photoluminescencematerial (e.g. CASN) on a back face of the substrate, therebysubstantially reducing costs while increasing brightness.

The LED-filament can comprise a single-layer structure comprising alayer comprising a mixture of the narrow-band red photoluminescencematerial and the first broad-band green to red photoluminescencematerials. To further reduce narrow-band red photoluminescence materialusage, the layer can further comprise particles of a light scatteringmaterial such as for example particles of zinc oxide; silicon dioxide;titanium dioxide; magnesium oxide; barium sulfate; aluminum oxide andcombinations thereof. A single-layer structure may be more robust andalso enhance ease of manufacture due the different photoluminescencematerials being comprised in the same layer. This may reduce cost andtime of manufacture, and also help eradicate errors during manufacturesince there are less steps involved in the creation of the single-layerstructure.

Alternatively, in order to further improve the blue absorptionefficiency of the narrow-band red photoluminescence material, theLED-filament can comprise a double-layer structure in which thenarrow-band red photoluminescence material is located in a separatelayer from the broad-band green to red photoluminescence materials withthe separate layer being disposed on top of the LED chips in, forexample, the form of a conformal coating. In such embodiments, anLED-filament can comprise a first layer comprising the first narrow-bandred photoluminescence material disposed on the plurality of blue LEDchips, and a second layer comprising the first broad-band green to redphotoluminescence material disposed on the first layer. In embodiments,the first layer can comprise a uniform thickness layer (film) on atleast the principle light emitting face of at least one of the LEDchips, that is the LED-filament comprises CSP (Chip Scale Packaged) LEDscontaining the narrow-band red photoluminescence material. The firstlayer can comprise a uniform thickness layer on all light emitting facesof the LED chips in the form of a conformal coating layer. To furtherreduce narrow-band red photoluminescence material usage, the first layercan further comprise particles of a light scattering material such asfor example particles of zinc oxide; silicon dioxide; titanium dioxide;magnesium oxide; barium sulfate; aluminum oxide and combinationsthereof. The inventors have discovered that compared with a LED-filamentcomprising a single-layer structure, a double-layer structure canprovide a substantial further reduction, up to 80% by weight reduction,in manganese-activated fluoride red photoluminescence material usage.Compared with known LED-filaments having manganese-activated fluoridered photoluminescence material on both front and back faces adouble-layer structure can provide a 90% by weight reduction inmanganese-activated fluoride red photoluminescence material usage. Byproviding the narrow-band red photoluminescence material in a respectivelayer disposed on the plurality of LED chips this increases theconcentration of narrow-band red photoluminescence material in immediateproximity to LED chips and improves the blue absorption efficiency ofthe narrow-band red photoluminescence material, thereby reducingnarrow-band red photoluminescence material usage.

In embodiments, where the first broad-band green to redphotoluminescence materials comprises a first broad-band redphotoluminescence material, a content ratio of the first broad-band redphotoluminescence material with respect to the total of the firstnarrow-band red photoluminescence material and the first broad-band redphotoluminescence material is at least one of: at least 20 wt %; atleast 30 wt %; at least 40 wt %; and in a range from about 20 wt % toless than 60 wt %.

The narrow-band red photoluminescence material(s) such as amanganese-activated fluoride red photoluminescence material can have apeak emission wavelength ranging from 630 nm to 633 nm and may compriseat least one of: K₂SiF₆:Mn⁴⁺(KSF), K₂GeF₆:Mn⁴⁺(KGF), andK₂TiF₆:Mn⁴⁺(KTF).

At least one of the first broad-band green to red photoluminescencematerial and the second broad-band green to red photoluminescencematerials can comprise a rare-earth-activated red photoluminescencematerial. The rare-earth-activated red photoluminescence materials canhave a peak emission wavelength ranging from 620 nm to 650 nm and maycomprise at least one of a nitride-based phosphor material having ageneral composition AAlSiN₃:Eu²⁺ where A is at least one of Ca, Sr orBa; a sulfur-based phosphor material having a general composition(Ca_(1-x)Sr_(x))(Se_(1-y)S_(y)):Eu²⁺ where 0≤x≤1 and 0<y≤1 and asilicate-based phosphor material having a general composition(Ba_(1-x)Sr_(x))₃SiO₅:Eu²⁺ where 0≤x≤1.

In embodiments, the first broad-band green to red photoluminescencematerials comprises a first broad-band green photoluminescence materialand the second broad-band green to red photoluminescence materialscomprises a second broad-band green photoluminescence material. Thefirst broad-band green photoluminescence material can have a peakemission wavelength ranging from 530 nm to 550 nm while the secondbroad-band green photoluminescence material can have a peak emissionwavelength ranging from 520 nm to 540 nm. The first and/or secondbroad-band green photoluminescence materials can comprise acerium-activated garnet phosphor having a general composition(Lu_(1-x)Y_(x))₃(Al_(1-y)Ga_(y))₅O₁₂:Ce where 0≤x≤1 and 0≤y≤1.

The partially light-transmissive substrate can comprise a materialselected from the group consisting of: alumina, silica, magnesium oxide,sapphire, quartz glass, diamond, silicon oxide and mixtures thereof.

The LED-filament can be operable to generate white light with acorrelated color temperature from 2700K to 6500K. The LED-filament canbe operable to generate white light with a general color rendering indexCRI Ra of at least 80 and optionally at least 90.

According to further embodiments, an LED-filament comprises: a partiallylight-transmissive substrate; a plurality of blue LED chips mounted on afront face of the substrate; a broad-band green photoluminescencematerial, a broad-band red photoluminescence material, and a narrow-bandmanganese-activated fluoride red photoluminescence material covering theplurality of blue LED chips and the front face of the substrate; andwherein a content ratio of the broad-band red photoluminescence materialwith respect to the total of the narrow-band red photoluminescencematerial and broad-band red photoluminescence material is at least 20 wt%. In embodiments, a content ratio of the broad-band redphotoluminescence material with respect to the total of the narrow-bandmanganese-activated fluoride red photoluminescence material andbroad-band red photoluminescence material is at least one of: at least30 wt %; and at least 40 wt %.

In embodiments, the LED-filament can comprise a first layer having thenarrow-band red photoluminescence material disposed on the plurality ofblue LED chips; and a second layer having the broad-band greenphotoluminescence material disposed on the first layer; and thebroad-band red photoluminescence material is in at least one of: thefirst layer and the second layer.

In further embodiments, LED-filaments can comprise a double-layerstructure on the front and back faces of the substrate, a so called“double-sided double-layer” structure. According to embodiments, anLED-filament comprises: a partially light-transmissive substrate; aplurality of blue LED chips mounted on a front face of the substrate; afirst photoluminescence layer comprising a first narrow-bandmanganese-activated fluoride red photoluminescence material disposed onthe plurality of blue LED chips; a second photoluminescence layercomprising a first broad-band green to red photoluminescence materialsdisposed on the first photoluminescence layer; a third photoluminescencelayer comprising a second narrow-band manganese-activated fluoride redphotoluminescence material disposed on the back face of the substrate;and a fourth photoluminescence layer comprising a second broad-bandgreen to red photoluminescence material disposed on the thirdphotoluminescence layer. In embodiments, the first layer can comprise auniform thickness layer (film) on at least the principle light emittingface of at least one of the LED chips, that is the LED-filamentcomprises CSP (Chip Scale Packaged) LEDs containing the narrow-band redphotoluminescence material. The first layer can comprise a uniformthickness layer on all light emitting faces of the LED chips in the formof a conformal coating layer. To reduce narrow-band redphotoluminescence material usage, at least one of the firstphotoluminescence layer and the third photoluminescence layer furthercomprises particles of a light scattering material selected from thegroup comprising: zinc oxide; silicon dioxide; titanium dioxide;magnesium oxide; barium sulfate; aluminum oxide; and combinationsthereof. The inventors have discovered such a double-sided double-layerstructure can substantially reduce (as much as 80% by weight for asubstrate with a transmittance of 100%) the usage amount of thenarrow-band red photoluminescence material compared with knownLED-filaments comprising narrow-band and broad-band redphotoluminescence materials on front and back faces of the substrate. Insuch embodiments, the substrate can have a transmittance in a range from20% to 100%.

According to a further embodiment, an LED-filament comprises: apartially light-transmissive substrate; a plurality of blue LED chipsmounted on a front face of the substrate; first broad-band green to redphotoluminescence materials and a narrow-band manganese-activatedfluoride red photoluminescence material covering the plurality of blueLED chips and the front face of the substrate; and second broad-bandgreen to red photoluminescence materials covering the back face of thesubstrate, wherein at least 70% of the total blue light generated by theLED chips is on the front face side of the substrate.

According to an aspect of the invention, an LED-filament lamp comprises:a light-transmissive envelope; and at least one LED-filament asdescribed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects and features of the present invention willbecome apparent to those ordinarily skilled in the art upon review ofthe following description of specific embodiments of the invention inconjunction with the accompanying figures, in which:

FIGS. 1A and 1B respectively illustrate partial cross-sectional A-A sideand plan views of a four LED-filament A-Series (A19) lamp in accordancewith an embodiment of the invention;

FIGS. 2A, 2B and 2C respectively illustrate schematic cross-sectionalB-B side, partial cutaway plan and cross-sectional C-C end views of asingle-layer LED-filament in accordance with an embodiment of theinvention for use in the lamp of FIGS. 1A and 1B;

FIGS. 3A and 3B are respectively a schematic representation of an LEDchip showing its emission characteristics in forward and backwarddirections and a schematic exploded representation of an LED chip andsubstrate indicating the distribution of blue excitation light presentat front and back face sides of the substrate;

FIGS. 4A and 4B are schematic cross-sectional end views of double-layerLED-filaments in accordance with embodiments of the invention;

FIGS. 5A and 5B are schematic cross-sectional end views of double-sideddouble-layer LED-filaments in accordance with embodiments of theinvention; and

FIG. 6 is a schematic cross-sectional end view of an LED-filament inaccordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will now be described in detailwith reference to the drawings, which are provided as illustrativeexamples of the invention so as to enable those skilled in the art topractice the invention. Notably, the figures and examples below are notmeant to limit the scope of the present invention to a singleembodiment, but other embodiments are possible by way of interchange ofsome or all of the described or illustrated elements. Moreover, wherecertain elements of the present invention can be partially or fullyimplemented using known components, only those portions of such knowncomponents that are necessary for an understanding of the presentinvention will be described, and detailed descriptions of other portionsof such known components will be omitted so as not to obscure theinvention. In the present specification, an embodiment showing asingular component should not be considered limiting; rather, theinvention is intended to encompass other embodiments including aplurality of the same component, and vice-versa, unless explicitlystated otherwise herein. Moreover, applicants do not intend for any termin the specification or claims to be ascribed an uncommon or specialmeaning unless explicitly set forth as such. Further, the presentinvention encompasses present and future known equivalents to the knowncomponents referred to herein by way of illustration. FIGS. 1A and 1Brespectively illustrate a partial cross-sectional side view through A-Aand a partial cutaway plan view of an LED-filament A-Series lamp (bulb)100 formed in accordance with an embodiment of the invention. TheLED-filament lamp (bulb) 100 is intended to be an energy efficientreplacement for a traditional incandescent A19 light bulb and can beconfigured to generate 550 lm of light with a CCT (Correlated ColorTemperature) of 2700 K and a general color rendering index CRI Ra of atleast 80. The LED-filament lamp is nominally rated at 4 W. As is known,an A-series lamp is the most common lamp type and an A19 lamp is 2⅜inches (19/8 inches) wide at its widest point and approximately 4⅜inches in length.

The LED-filament lamp 100 comprises a connector base 102, alight-transmissive envelope 104; an LED-filament support 106 and fourLED-filaments 108 a, 108 b, 108 c, 108 d.

In some embodiments, the LED-filament lamp 100 can be configured foroperation with a 110V (r.m.s.) AC (60 Hz) mains power supply as used inNorth America. For example and as illustrated, the LED-filament lamp 100can comprise an E26 (ϕ26 mm) connector base (Edison screw lamp base) 102enabling the lamp to be directly connected to a mains power supply usinga standard electrical lighting screw socket. It will be appreciated thatdepending on the intended application other connector bases can be usedsuch as, for example, a double contact bayonet connector (i.e. B22d orBC) as is commonly used in the United Kingdom, Ireland, Australia, NewZealand and various parts of the British Commonwealth or an E27 (ϕ27 mm)screw base (Edison screw lamp base) as used in Europe. The connectorbase 102 can house rectifier or other driver circuitry (not shown) foroperating the LED-filament lamp.

The light-transmissive envelope 104 is attached to the connector 102.The light-transmissive envelope 104 and LED-filament support 106 cancomprise glass. The envelope 104 defines a hermetically sealed volume110 in which the LED-filaments 108 a to 108 d are located. The envelope104 can additionally incorporate or include a layer of a light diffusive(scattering) material such as for example particles of zinc oxide (ZnO),titanium dioxide (TiO₂), barium sulfate (BaSO₄), magnesium oxide (MgO),silicon dioxide (SiO₂) or aluminum oxide (Al₂O₃).

The LED-filaments 108 a to 108 d, which are linear (strip or elongate)in form, are oriented such that their direction of elongation isgenerally parallel to an axis 112 of the lamp 100. In this embodiment,the LED-filaments 108 a to 108 d are equally circumferentially spacedaround the glass filament support 106 (FIG. 1B), although it will beappreciated that in other embodiments the LED-filaments may not beequally spaced around the glass support. A first electrical contact 114a to 114 d on a first end of each LED-filament 108 a to 108 d distal tothe connector base 102 is electrically and mechanically connected to afirst conducting wire 116 that passes down an axis of the LED filamentsupport 106 to the connector base 102. A second electrical contact 118 ato 118 d on a second end of each LED-filament 108 a to 108 d proximal tothe connector base 102 is electrically and mechanically connected to asecond conducting wire 120 that passes through a base portion 122 of theLED filament support 106 to the connector base 102. As illustrated, theLED filaments 108 a to 108 d can be electrically connected in parallel.

An LED-filament according to an embodiment of the invention is nowdescribed with reference to FIGS. 2A, 2B and 2C which respectively showa cross-sectional side view through B-B, a partial cut-away plan and across-sectional C-C end view of a single-layer LED-filament 208.Throughout this specification, like reference numerals preceded by thefigure number are used to denote like parts. The LED-filament 208comprises a partially light-transmissive substrate 224 having an arrayof blue emitting (465 nm) unpackaged LED chips (dies) 226 mounteddirectly to a front (first) face 228. Typically each LED-filament has atotal nominal power of about 0.7 to 1 W.

The substrate 224 can further comprise the respective electricalcontacts 214, 218 on the front face 228 at the first and second ends ofthe substrate 224 for electrical connection to a respective one of theconducting wires 116, 120 (FIG. 1A) to provide electrical power tooperate the LED-filament. The electrical contacts 214, 218 can comprisecopper, silver or other metal or a transparent electrical conductor suchas indium tin oxide (ITO). In the embodiment, illustrated the substrate224 is planar and has an elongate form (strip) with the LED chips 226being configured as a linear array (string) and equally spaced along thelength (direction of elongation) of the substrate. As indicated in FIGS.2A and 2B the LED chips 226 can be electrically connected in series bybond wires 230 between adjacent the LED chips of the string and wirebonds 232 between the LED chips at the distal ends of the substrate andtheir respective electrical contact 214, 218.

When the LED-filament 208 is used as a part of an energy efficient bulban elongate configuration is typically preferred since the appearanceand emission characteristics of the device more closely resembles atraditional filament of an incandescent bulb. It should be noted thatthe LED chips 226 are unpackaged and emit light from both their top andbottom (base) faces with the base surface of the LED chip mounteddirectly on the substrate 224.

In accordance the invention, the light-transmissive substrate 224 cancomprise any material which is partially light-transmissive andpreferably has a transmittance to visible light from 2% to 70%(reflectance of 98% to 30%). The substrate can comprise a glass, ceramicmaterial or a plastics material such as polypropylene, silicone or anacrylic. Typically, in embodiments the light-transmissive substratecomprises a porous ceramic substrate composed of alumina that has atransmittance of about 40%. To aid in the dissipation of heat generatedby the LED chips 226, the substrate 224 can not only belight-transmissive, but can also be thermally conductive to aid in thedissipation of heat generated by the LED chips. Examples of suitablelight-transmissive thermally conductive materials include: magnesiumoxide, sapphire, aluminum oxide, quartz glass, and diamond. Thetransmittance of the thermally conductive substrate can be increased bymaking the substrate thin. To increase mechanical strength, thesubstrate can comprise a laminated structure with the thermallyconductive layer mounted on a light-transmissive support such as a glassor plastics material. To further assist in the dissipation of heat, thevolume 110 (FIG. 1A) within the glass envelope 104 (FIG. 1A) ispreferably filled with a thermally conductive gas such as helium,hydrogen or a mixture thereof.

In accordance with embodiments of the invention, the LED-filament 208further comprises a first photoluminescence wavelength conversionmaterial 236 applied to and covering the LED chips 226 and front face228 of the substrate 224 and a second different photoluminescencewavelength conversion material 238 applied to and covering the secondback (opposite) face 234 of the substrate 224. The firstphotoluminescence wavelength conversion material 236 is applied directlyto the LEDs chips 226 and covers the front face of the substrate in theform of an encapsulating layer.

In accordance with the invention, the first photoluminescence wavelengthconversion material 236 comprises a mixture of a first broad-band greenphotoluminescence material having a peak emission wavelength rangingfrom 520 nm to 560 nm (preferably 540 nm to 545 nm), a first broad-bandred photoluminescence material having a peak emission wavelength rangingfrom 620 nm to 650 nm and a narrow-band red photoluminescence materialtypically a manganese-activated fluoride phosphor. Collectively, thefirst broad-band green and red photoluminescence materials will bereferred to as first broad-band green to red photoluminescencematerials. Since in this embodiment both the narrow-band red andbroad-band green to red photoluminescence materials are provided as amixture in a single layer the LED-filament will be referred to as a“single-layer” structured filament.

The second photoluminescence wavelength conversion material 238comprises a mixture of only a second broad-band green photoluminescencematerial having a peak emission wavelength ranging from 520 nm to 560 nm(preferably 520 nm to 540 nm) and a second broad-band red(non-manganese-activated fluoride) photoluminescence material having apeak emission wavelength ranging from 620 nm to 650 nm. Collectively,the second broad-band green and red photoluminescence materials will bereferred to as second broad-band green to red photoluminescencematerials.

In contrast, in known LED-filaments, the same photoluminescence materialcomposition (narrow-band and broad-band red photoluminescence materials)is provided on the front and back faces of the filament. Suitablebroad-band green photoluminescence materials, narrow-band redphotoluminescence materials and broad-band red photoluminescencematerials are discussed below.

In the embodiment illustrated in FIGS. 2A and 2B, the first and secondphotoluminescence conversion materials 236 and 238 are constituted as asingle layer comprising a mixture of broad-band green and redphotoluminescence materials.

In operation, blue excitation light generated by the LED chips 210excites the green-emitting and red emitting photoluminescence materialsto generate green and red light. The emission product of theLED-filament 208 which appears white in color comprises the combinedphotoluminescence light and unconverted blue LED light. Since thephotoluminescence light generation process is isotropic, phosphor lightis generated equally in all directions and light emitted in a directiontowards the substrate 224 can pass through the substrate and be emittedfrom the back of the LED-Filament 208. It will be appreciated that theuse of a partially light-transmissive substrate 224 enables theLED-filament to achieve an emission characteristic in which light isemitted in a direction away from both the front face 228 and back face234 of the substrate. Additionally, particles of a light scatteringmaterial can be combined with the phosphor material to reduce thequantity of phosphor required to generate a given emission productcolor.

FIG. 3A is a schematic representation of an LED chip 326 showing itsemission characteristics in forward/upward 340 and backward/downward 342directions and FIG. 3B is a schematic exploded representation of the LEDchip 326 and a partially light-transmissive substrate 324 indicating thedistribution of blue excitation light on opposite face sides 328 and 334of the substrate 324.

Referring to FIG. 3A and assuming that the blue LED chip 326 emits equalamounts of blue light from its top surface 344 and its base 346, then50% of the total blue light generated by the LED chip is emitted in aforward direction 340 away from the front face of the substrate and 50%of the total blue light generated by the LED chip is emitted in abackward direction 342 towards the front face of the substrate.Referring to FIG. 3B and assuming that the partially light-transmissivesubstrate 324 has a transmittance of 40% and a reflectance of 60%, only40% of the blue light 342 (i.e. 20% of the total blue light generated bythe blue LED chip 346) will pass through the substrate 324 and emanate346 from the back face side 334 of the substrate 324. The remaining 60%of blue light 342 (i.e. 30% of the total blue light generated by theblue LED chip) will be reflected by the substrate 324 in a forwarddirection and emanate from the front face side 328 of the substrate. Itwill appreciated that the net effect is that approximately 80% of thetotal blue light generated by the blue LED chip 348 will be on (emanatesfrom) the front face side of the substrate and only 20% of the totalblue light generated by the blue LED chip 348 will be on (emanates from)the back face side of the substrate. Clearly, when the photoluminescencematerials are present these figures may change due to scattering of bluelight by the photoluminescence materials. As described above, byconfiguring the proportion of total blue excitation light generated bythe blue LED chips present at the front face side of the substrate to besubstantially greater (typically at least 70% of the total) than at theopposite back face side this enables use of the higher brightnessmanganese-activated fluoride phosphor on the front face of the substrateonly and a less expensive non manganese-activated fluoride phosphor onthe back face of the substrate while still providing substantially mostof the increase in brightness benefit but using only half (50% byweight) the quantity of manganese-activated fluoride photoluminescencematerial. TABLE 1 tabulates the effect of substratetransmittance/reflectance on the proportion of total blue excitationlight on (emanates from) the front face and back face sides of thesubstrate and the relative overall brightness of the LED-filament. Thedata assumes that each blue LED chip generates equal amounts of blueexcitation light in forward and backward directions. The overallrelative brightness is relative to a known LED-filament having CASN onthe front and back faces of the substrate. For comparison, the relativebrightness for an LED-filament having KSF on both faces of the substrateis 120% though it will be appreciated that uses twice the amount of KSFthan the LED-filaments of the invention.

TABLE 1 Effect of substrate transmittance/reflectance on the proportionof blue excitation light on the front and back face sides of thesubstrate and LED-filament brightness % of total blue Substrateexcitation light on: LED- Trans- Front face Back face Filament mittanceReflectance side side Brightness (%) (%) of substrate of substrate (%) 595 97.5 2.5 124.4 10 90 95.0 5.0 123.8 20 80 90.0 10.0 122.5 40 60 80.020.0 120.0 50 50 75.0 25.0 118.8 60 40 70.0 30.0 117.5 70 30 65.0 35.0116.3

FIGS. 4A and 4B are schematic cross-sectional end views of double-layerLED-filaments 408 in accordance with embodiments of the invention. Inthese embodiments, the first photoluminescence wavelength conversionmaterial 436 covering the LED chips comprises a “double-layer” structurecomprising first and second photoluminescence layers 450 and 452 thatrespectively contain narrow-band red and first broad-band green to redphotoluminescence materials. As illustrated in FIGS. 4A and 4B, thefirst photoluminescence layer 450, containing the narrow-band redphotoluminescence material, is disposed on and covers the LED chips 426and the second photoluminescence layer 452, containing the firstbroad-band green to red photoluminescence materials (that is firstbroad-band green and first broad-band red photoluminescence materials),is disposed on and covers the first photoluminescence layer 450 (that isthe first photoluminescence layer 450 is in closer proximity to the LEDchips than the second photoluminescence layer).

The double-layer LED-filament of FIG. 4A can be manufactured by firstlydepositing the first photoluminescence layer 450 onto the LED chips 426and then depositing the second photoluminescence layer 452 on the firstphotoluminescence layer 450. As illustrated the first photoluminescencelayer 450 can have a cross section that is generally semi-circular inprofile.

In the double-layer LED-filament of FIG. 4B the first photoluminescencelayer 450 comprises a uniform thickness coating layer that is applied tothe light emitting faces of individual LED chips. LED chips with auniform thickness layer (film) of phosphor on their light emitting facesare often referred to as CSP (Chip Scale Packaged) LEDs. As illustratedin FIG. 4B the LED chip 426 has a uniform thickness layer applied to thetop light emitting and four side light emitting faces and is in the formof a conformal coating. In embodiments (not shown) the LED chip 426 hasa uniform thickness first photoluminescence layer 450 applied to theprinciple (top) light emitting face only. The double-layer LED-filamentcan be manufactured by first applying the first photoluminescence layer450 to at least the principle light emitting face of individual LEDchips 426, for example using a uniform thickness (typically 20 μm to 300μm) photoluminescence film comprising the narrow-band redphotoluminescence material. The LED chips 426 are then mounted on thesubstrate 424 and the second photoluminescence layer 452 then depositedto cover the substrate and LED chips. Compared with the double-layerLED-filament of FIG. 4A a uniform thickness coating layer can bepreferred as it concentrates all of the narrow-band redphotoluminescence material as close to the LED chip as possible andensures that, regardless of physical location within the layer, all ofthe narrow-band red photoluminescence material receives exposure tosubstantially the same excitation light photon density. Such anarrangement can maximize the reduction in narrow-band redphotoluminescence material usage.

The inventors have discovered that providing the narrow-band redphotoluminescence material as a respective individual layer 450(double-layer structure) is found to further substantially reduce (up toa further 80% by weight reduction) the usage amount of the narrow-bandred photoluminescence material compared with an LED-filament in whichthe narrow-band red and broad-band green photoluminescence materialscomprise a mixture in a single layer (FIG. 2C). Moreover, compared witha known LED-filament in which narrow-band red photoluminescence materialis provided on both faces of the substrate, a double-layer structuredLED-filament reduces the usage amount of narrow-band redphotoluminescence material by as much as 90% by weight.

It is believed that the reason for this reduction in usage amount, isthat in an LED-filament (FIG. 2C) in which the photoluminescencematerial 236 comprises a single photoluminescence layer comprising amixture of a narrow-band red photoluminescence material and broadbandgreen to red photoluminescence materials, the photoluminescencematerials have equal exposure to blue excitation light. Sincenarrow-band red photoluminescence materials, especiallymanganese-activated fluoride photoluminescence materials, have a muchlower blue light absorption capability than the broad-band greenphotoluminescence materials a greater amount of narrow-band redphotoluminescence material is necessary to convert enough blue light tothe required red emission. By contrast, in the LED-filaments 408 ofFIGS. 4A and 4B, the narrow-band red photoluminescence material in itsseparate respective layer 450 is exposed to blue excitation lightindividually; thus, more of the blue excitation light can be absorbed bythe narrow-band red photoluminescence material and the remaining blueexcitation light can penetrate through to the second photoluminescencelayer 452 containing the broad-band green to red photoluminescencematerials. Advantageously, in this structure the narrow-band redphotoluminescence material can more effectively convert the blueexcitation light to red emission without competition from the green tored photoluminescence materials. Therefore, the amount (usage) of anarrow-band red photoluminescence material required to achieve a targetcolor point can be reduced compared with LED-filaments comprising asingle-layer comprising a mixture of photoluminescence materials.

FIGS. 5A and 5B are schematic cross-sectional end views of adouble-sided double-layer LED-filaments in accordance with embodimentsof the invention. In these embodiments, both the first 536 and second538 photoluminescence wavelength conversion materials covering the frontand back face of the substrate comprise a “double-layer” structure. Onthe front face of the substrate, the first photoluminescence material536 covering the LED chips comprises first and second photoluminescencelayers 550 and 552 that respectively contain first narrow-band red andfirst broad-band green to red photoluminescence materials. Asillustrated, the first photoluminescence layer 550, containing the firstnarrow-band red photoluminescence material, is disposed on and coversthe LED chips 526 and the second photoluminescence layer 552, containingthe first broad-band green to red photoluminescence materials, isdisposed on and covers the first photoluminescence layer 550 (that isthe first photoluminescence layer 550 is in closer proximity to the LEDchips than the second photoluminescence layer). On the back face of thesubstrate, the second photoluminescence material 538 covering the backface of the substrate 524 comprises third and fourth photoluminescencelayers 554 and 556 that respectively contain second narrow-band red andsecond broad-band green to red photoluminescence materials. Asillustrated, the third photoluminescence layer 554, containing thesecond narrow-band red photoluminescence material, is disposed on andcover a part of the substrate corresponding with the LED chips 526 andthe fourth photoluminescence layer 556, containing the second broad-bandgreen to red photoluminescence materials, is disposed on and covers thethird photoluminescence layer 554 (that is the third photoluminescencelayer 554 is in closer proximity to the back face of the substrate thanthe fourth photoluminescence layer).

The double-layer double-sided LED-filament of FIG. 5A can bemanufactured by firstly depositing the first photoluminescence layer 550onto the LED chips 526 and then depositing the second photoluminescencelayer 552 on the first photoluminescence layer 550. As illustrated thefirst photoluminescence layer 550 can have a cross section that isgenerally semi-circular in profile. The third photoluminescence layer554 is deposited on the back face of the substrate corresponding withthe LED chips 526, for example as a strip, and the fourthphotoluminescence layer 556 then deposited on and covers the thirdphotoluminescence layer 554. As illustrated the third photoluminescencelayer 554 can have a cross section that is generally semi-circular inprofile.

In the double-layer double-sided LED-filament of FIG. 5B the firstphotoluminescence layer 550 comprises a uniform thickness layer appliedto at least the principle light emitting face of individual LED chips,that is the LED-filament comprises CSP LEDs. As illustrated in FIG. 5Bthe LED chip 526 has a uniform thickness layer applied to the top lightemitting and four side light emitting faces and is in the form of aconformal coating. In embodiments (not shown) the LED chip 526 has auniform thickness first photoluminescence layer 550 applied to theprinciple (top) light emitting face only. The double-layer LED-filamentcan be manufactured by first applying the first photoluminescence layer550 to at least the principle light emitting face of individual LEDchips 526, for example using a uniform thickness (typically 20 μm to 300μm) photoluminescence film comprising the narrow-band redphotoluminescence material. The LED chips 526 are then mounted on thesubstrate 524 and the second photoluminescence layer 552 then depositedto cover the substrate and LED chips. The third photoluminescence layer554 is deposited of the back face of the substrate corresponding withthe LED chips 526, for example as a strip, and the fourthphotoluminescence layer 556 then deposited on, and covers, the thirdphotoluminescence layer 554.

The inventors have discovered LED-filaments having a double-sideddouble-layer structure can substantially reduce (as much as 80% byweight reduction for a substrate with a transmittance of 100%) the usageamount of the narrow-band red photoluminescence material compared withknown LED-filaments comprising narrow-band and broad-band redphotoluminescence materials on front and back faces.

FIG. 6 is a schematic cross-sectional end view of an LED-filament inaccordance with an embodiment of the invention. In this embodiment, oneor more of the LED chips 624 has a reflector 660 on its base. Thereflector 660 reduces blue light emission from the base of the LED chipand reflect such light in a forward/upward direction. The reflector canbe 100% light reflective or partially light reflective. It will beappreciated that the invention contemplates that other embodimentsdisclosed herein may also include reflector(s) on the base(s) of the LEDchips.

In various embodiments of the invention, and to reduce photoluminescencematerial usage, in particular to further reduce narrow-band redphotoluminescence material usage, the LED-filament can further compriseparticles of a light scattering material such as for example particlesof zinc oxide (ZnO), titanium dioxide (TiO₂) barium sulfate (BaSO₄),magnesium oxide (MgO), silicon dioxide (SiO₂), aluminum oxide (Al₂O₃),zirconium dioxide (ZrO₂) or mixtures thereof. The particles of lightscattering material can be provided as a mixture with any of thephotoluminescence materials and/or in a separate layer in contact with aphotoluminescence material layer. Preferably, the particles of lightscattering material are incorporated with the narrow-band redphotoluminescence material to further reduce narrow-band redphotoluminescence usage. For example, for a single-layer structuredLED-filament the particles of light scattering material can beincorporated in the first photoluminescence wavelength conversionmaterial 236 as part of the mixture of the first broad-band green to redphotoluminescence materials and the narrow-band red photoluminescencematerial (FIG. 2C). For a double-layer structured LED filament, theparticles of light scattering material can be incorporated as a mixturewith the narrow-band red photoluminescence material in the firstphotoluminescence layer 450 (FIG. 4). For a double-sided double-layerLED-filament, the particles of light scattering material can beincorporated as a mixture with the narrow-band red photoluminescencematerial in the first and/or third photoluminescence layers 550, 554(FIG. 5).

Alternatively and/or in addition, the particles of light scatteringmaterial can be provided in a separate layer that is in contact with thelayer containing the narrow-band red photoluminescence material tofurther reduce narrow-band red photoluminescence usage.

The inclusion of particles of a light scattering material with thephotoluminescence material increases the number of collisions of LEDgenerated excitation light with particles of the photoluminescencematerial enhancing photoluminescence light generation which decreasesthe amount of photoluminescence material usage. It is believed that onaverage as little as 1 in 10,000 interactions of a photon with aphotoluminescence material results in absorption and generation ofphotoluminescence light. The majority, about 99.99%, of interactions ofphotons with a photoluminescence material particle result in scatteringof the photon. Since the inclusion of the light scattering materialsincreases the number of collisions this increases the probability ofphotoluminescence light generation, which decreases the amount ofphotoluminescence material usage to generate a selected emissionintensity.

Broad-Band Green Photoluminescence Materials

In this patent specification, a broad-band green photoluminescencematerial refers to a material which generates light having a peakemission wavelength (λ_(pe)) in a range ˜520 nm to ˜560 nm, that is inthe yellow/green to green region of the visible spectrum. Preferably,the green photoluminescence material has a broad emission characteristicand preferably has a FWHM (Full Width Half Maximum) of between about 50nm and about 120 nm. The green photoluminescence material can compriseany photoluminescence material, such as for example, garnet-basedinorganic phosphor materials, silicate phosphor materials and oxynitridephosphor materials. Examples of suitable green phosphors are given inTABLE 2.

In some embodiments, the green photoluminescence materials comprises acerium-activated yttrium aluminum garnet phosphor of general compositionY₃(Al_(1-y)Ga_(y))₅O₁₂:Ce (YAG) where 0<y<1 such as for example a YAGseries phosphor from Intematix Corporation, Fremont Calif., USA whichhave a peak emission wavelength of in a range 520 nm to 543 nm and aFWHM of ˜120 nm. In this patent specification, the notation YAG#represents the phosphor type—YAG-based phosphors—followed by the peakemission wavelength in nanometers (#). For example, YAG535 denotes a YAGphosphor with a peak emission wavelength of 535 nm. The greenphotoluminescence material may comprise a cerium-activated yttriumaluminum garnet phosphor of general composition (Y,Ba)₃(Al,Ga)₅O₁₂:Ce(YAG) such as for example a GNYAG series phosphor from IntematixCorporation, Fremont Calif., USA. In some embodiments, the greenphotoluminescence material can comprise an aluminate (LuAG) phosphor ofgeneral composition Lu₃Al₅O₁₂:Ce (GAL). Examples of such phosphorsinclude for example the GAL series of phosphor from IntematixCorporation, Fremont Calif., USA which have a peak emission wavelengthof 516 nm to 560 nm and a FWHM of ˜120 nm. In this patent specification,the notation GAL# represents the phosphor type (GAL)—LuAG-basedphosphors—followed by the peak emission wavelength in nanometers (#).For example, GAL520 denotes a GAL phosphor with a peak emissionwavelength of 520 nm. Suitable green phosphors are given in TABLE 2.

Examples of green silicate phosphors include europium activatedortho-silicate phosphors of general composition (Ba, Sr)₂SiO₄:Eu such asfor example G, EG, Y and EY series of phosphors from IntematixCorporation, Fremont Calif., USA which have a peak emission wavelengthin a range 507 nm to 570 nm and a FWHM of ˜70 nm to ˜80 nm. Suitablegreen phosphors are given in TABLE 2.

In some embodiments, the green phosphor can comprise a green-emittingoxynitride phosphor as taught in U.S. Pat. No. 8,679,367 entitled“Green-Emitting (Oxy) Nitride-Based Phosphors and Light Emitting DevicesUsing the Same” which is hereby incorporated in its entirety. Such agreen-emitting oxynitride (ON) phosphor can have a general compositionEu²⁺:M²⁺Si₄AlO_(x)N_((7-2x/3)) where 0.1≤x≤1.0 and M²⁺ is one or moredivalent metal selected from the group consisting of Mg, Ca, Sr, Ba, andZn. In this patent specification, the notation ON# represents thephosphor type (oxynitride) followed by the peak emission wavelength(λ_(pe)) in nanometers (#). For example ON495 denotes a green oxynitridephosphor with a peak emission wavelength of 495 nm.

TABLE 2 Example broad-band green photoluminescence materials GeneralWavelength Phosphor Composition λ_(p) (nm) YAGY_(3−x)(Al_(1−y)Ga_(y))₅O₁₂:Ce_(x) 0.01 < x < 0.2 & 520-550 (YAG#) 0 < y< 2.5 GNYAG (Y,Ba)_(3−x)(Al_(1−y)Ga_(y))₅O₁₂:Ce_(x) 0.01 < x < 0.2 &520-550 (YAG#) 0 < y < 2.5 LuAG Lu_(3−x)(Al_(1−y)M_(y))₅O₁₂:Ce_(x) 0.01< x < 0.2 & 500-550 (GAL#) 0 < y < 1.5 M = Mg, Ca, Sr, Ba, Ga, LuAGLu_(3−x)(Al_(1−y)Ga_(y))₅O₁₂:Ce_(x) 0.01 < x < 0.2 & 500-550 (GAL#) 0 <y < 1.5 Silicate A₂SiO₄:Eu A = Mg, Ca, Sr, Ba 500-550 Silicate(Sr_(1−x)Ba_(x))₂SiO₄:Eu 0.3 < x < 0.9 500-550 OxynitrideEu²⁺:M²⁺Si₄AlO_(x)N(_(7−2x/3)) M²⁺ = Mg, Ca, 500-550 (ON#) Sr, Ba, Zn0.1 ≤ x ≤ 1.0

Red Photoluminescence Materials

Narrow-Band Red Photoluminescence Materials

In this patent specification, a narrow-band red photoluminescencematerial refers to a photoluminescence material which, in response tostimulation by excitation light, generates light having a peak emissionwavelength in a range 610 nm to 655 nm; that is light in the red regionof the visible spectrum and which has a narrow emission characteristicwith a full width at half maximum (FWHM) emission intensity of betweenabout 5 nm and about 50 nm (less than about 50 nm). As described above,the narrow-band red photoluminescence can comprise a manganese-activatedfluoride red photoluminescence material that is disposed on and coversthe front face of the substrate on which the LED chips are mounted. Anexample of a narrow-band red manganese-activated fluoridephotoluminescence material is manganese-activated potassiumhexafluorosilicate phosphor (KSF)—K₂SiF₆:Mn⁴⁺(KSF). An example of such aKSF phosphor is NR6931 KSF phosphor from Intematix Corporation, FremontCalif., USA which has a peak emission wavelength of about 632 nm. Othermanganese-activated phosphors can include: K₂GeF₆:Mn⁴⁺(KGF) andK₂TiF₆:Mn⁴⁺(KTF).

Broad-Band Red Photoluminescence Materials

In this patent specification, a broad-band red photoluminescencematerial (also referred to as a non-manganese-activated fluoride redphotoluminescence material) refers to a photoluminescence materialwhich, in response to stimulation by excitation light, generates lighthaving a peak emission wavelength in a range 600 nm to 640 nm; that islight in the orange to red region of the visible spectrum and which hasa broad emission characteristic with a full width at half maximum (FWHM)emission intensity of greater than about 50 nm. As described above, thebroad-band red photoluminescence can comprise rare-earth activated redphotoluminescence materials. A broad-band red photoluminescence material(non-manganese-activated fluoride red photoluminescence material)denotes a red photoluminescence material whose crystal structure isother than that of a narrow-band red photoluminescence material(manganese-activated fluoride photoluminescence material), such as forexample rare-earth-activated red photoluminescence materials and cancomprise any such red photoluminescence material that is excitable byblue light and operable to emit light with a peak emission wavelengthλ_(p) in a range about 600 nm to about 640 nm. Rare-earth-activated redphotoluminescence material can include, for example, a europiumactivated silicon nitride-based phosphor, α-SiAlON, Group IIA/BBselenide sulfide-based phosphor or silicate-based phosphors. Examples ofred phosphors are given in TABLE 3.

In some embodiments, the europium activated silicon nitride-basedphosphor comprises a Calcium Aluminum Silicon Nitride phosphor (CASN) ofgeneral formula CaAlSiN₃:Eu²⁺. The CASN phosphor can be doped with otherelements such as strontium (Sr), general formula (Sr,Ca)AlSiN₃:Eu²⁺. Inthis patent specification, the notation CASN# represents the phosphortype (CASN) followed by the peak emission wavelength (λ_(pe)) innanometers (#). For example, CASN625 denotes a red CASN phosphor with apeak emission wavelength of 625 nm.

In an embodiment, the rare—earth-activated red phosphor can comprise ared-emitting phosphor as taught in U.S. Pat. No. 8,597,545 entitled“Red-Emitting Nitride-Based Calcium-Stabilized Phosphors” which ishereby incorporated in its entirety. Such a red emitting phosphorcomprises a nitride-based composition represented by the chemicalformula M_(a)Sr_(b)Si_(c)Al_(d)N_(e)Eu_(f), wherein: M is Ca, and0.1≤a≤0.4; 1.5<b<2.5; 4.0≤c≤5.0; 0.1≤d≤0.15; 7.5<e<8.5; and 0<f<0.1;wherein a+b+f>2+d/v and v is the valence of M.

Alternatively, the rare—earth-activated red phosphor can comprise a redemitting nitride-based phosphor as taught in U.S. Pat. No. 8,663,502entitled “Red-Emitting Nitride-Based Phosphors” which is herebyincorporated in its entirety. Such a red emitting phosphor comprising anitride-based composition represented by the chemical formulaM_((x/v))M′₂Si_(5-x)Al_(x)N₈:RE, wherein: M is at least one monovalent,divalent or trivalent metal with valence v; M′ is at least one of Mg,Ca, Sr, Ba, and Zn; and RE is at least one of Eu, Ce, Tb, Pr, and Mn;wherein x satisfies 0.1≤x<0.4, and wherein said red-emitting phosphorhas the general crystalline structure of M′₂Si₅N₈:RE, Al substitutes forSi within said general crystalline structure, and M is located withinsaid general crystalline structure substantially at the interstitialsites. An example of one such a phosphor is XR610 red nitride phosphorfrom Intematix Corporation, Fremont Calif., USA which has a peakemission wavelength of 610 nm.

Rare-earth-activated red phosphors can also include Group IIA/IMselenide sulfide-based phosphors. A first example of a Group IIA/IMselenide sulfide-based phosphor material has a compositionMSe_(1-x)S_(x):Eu, wherein M is at least one of Mg, Ca, Sr, Ba and Znand 0<x<1.0. A particular example of this phosphor material is CSSphosphor (CaSe_(1-x)S_(x):Eu). Details of CSS phosphors are provided inco-pending United States patent application Publication NumberUS2017/0145309 filed 30 Sep. 2016, which is hereby incorporated byreference in its entirety. The CSS red phosphors described in UnitedStates patent publication US2017/0145309 can be used in the presentinvention. The emission peak wavelength of the CSS phosphor can be tunedfrom 600 nm to 650 nm by altering the S/Se ratio in the composition andexhibits a narrow-band red emission spectrum with FWHM in the range ˜48nm to ˜60 nm (longer peak emission wavelength typically has a largerFWHM value). In this patent specification, the notation CSS# representsthe phosphor type (CSS) followed by the peak emission wavelength innanometers (#). For example, CSS615 denotes a CSS phosphor with a peakemission wavelength of 615 nm.

In some embodiments, the rare—earth-activated red phosphor can comprisean orange-emitting silicate-based phosphor as taught in U.S. Pat. No.7,655,156 entitled “Silicate-Based Orange Phosphors” which is herebyincorporated in its entirety. Such an orange-emitting silicate-basedphosphor can have a general composition (Sr_(1-x)M_(x))_(y)Eu_(z)SiO₅where 0<x≤0.5, 2.6≤y≤3.3, 0.001≤z≤0.5 and M is one or more divalentmetal selected from the group consisting of Ba, Mg, Ca, and Zn. In thispatent specification, the notation O# represents the phosphor type(orange silicate) followed by the peak emission wavelength (λ_(pe)) innanometers (#). For example, O600 denotes an orange silicate phosphorwith a peak emission wavelength of 600 nm.

TABLE 3 Example broad-band red photoluminescence materials GeneralWavelength Phosphor Composition λ_(p) (nm) CASN (Ca_(1−x)Sr_(x)) 0.5 < x≤ 1 600-650 (CASN#) AlSiN₃:Eu 258 nitride Ba_(2−x)Sr_(x)Si₅N₈:Eu 0 ≤ x ≤2 580-650 Group IIA/IIB M = Mg, Ca, Sr, Ba, Zn Selenide SulfideMSe_(1−x)S_(x):Eu 0 < x < 1.0 600-650 (CSS#) CSS CaSe_(1−x)S_(x):Eu 0 <x < 1.0 600-650 (CSS#) Silicate (Sr_(1−x)M_(x))_(y)Eu_(z)SiO₅ M = Ba,Mg, Ca, Zn 565-650 (O#) 0 < x ≤ 0.5 2.6 ≤ y ≤ 3.3 0.001 ≤ z ≤ 0.5

NOMENCLATURE

In this specification, the following nomenclature is used to denoteLED-filaments: Com.# denotes a comparative LED-filament having the samephotoluminescence materials on the front and back faces of the substrateand Dev.# denotes an LED-filament (device) in accordance with anembodiment of the invention having a narrow-band red(manganese-activated fluoride) photoluminescence material on the frontface of the substrate and a broad-band red photoluminescence material ona back face of the substrate.

Experimental Data—Single-Layer Structure LED-Filament

Comparative LED-filaments (Com.1 and Com.2) and single-layerLED-filament in accordance with the invention (Dev.1) each comprise a 52mm by 1.5 mm porous silica substrate with a transmittance ≈40% havingtwenty four serially connected 1025 (10 mil×25 mil) blue LED chips ofdominant wavelength λ_(d)=456 nm mounted on a front face. EachLED-filament is a nominal 0.7 W device and is intended to generate whitelight with a target Correlated Color Temperature (CCT) of 2700K and atarget general color rendering index CRI Ra of 90.

The photoluminescence materials (phosphors) used in the test devices areKSF phosphor (K₂SiF₆:Mn⁴⁺) from Intematix Corporation, CASN phosphor(Ca_(1-x)Sr_(x)AlSiN₃:Eu λ_(pe)≈640 nm), green YAG phosphor (IntematixNYAG4156—(Y, Ba)_(3-x)(Al_(1-y)Ga_(y))₅O₁₂:Ce_(x) Peak emissionwavelength λ_(pe)=550 nm) and green LuAG phosphor (IntematixGAL535-Lu_(3-x)(Al_(1-y)Ga_(y))₅O₁₂:Ce_(x) λ_(pe)≈535 nm).

The red and green phosphors were mixed in a phenyl silicone and themixture dispensed onto the front and back faces of the substrate.

TABLE 4 tabulates phosphor composition of comparative LED-filamentsCom.1 and Com.2 and an LED-filament Dev.1 in accordance with theinvention.

As can be seen from TABLE 4, in terms of phosphor composition:comparative LED-filament Com. 1 comprises the same phosphor compositionon the front and back faces of the substrate and comprises a mixture of7 wt % CASN640 and 93 wt % GAL535. Comparative LED-filament Com.2comprises the same phosphor composition on the front and back faces ofthe substrate and comprises a mixture of 60 wt % KSF and 40 wt % YAG550.LED-filament Dev.1, in accordance with the invention, comprises on thefront face of the substrate a mixture of 56 wt % KSF, 4 wt % CASN615 and40 wt % YAG550 and on a back face of the substrate a mixture of 7 wt %CASN and 93 wt % GAL535.

TABLE 4 Phosphor composition of comparative LED-filaments (Com.1 andCom.2) and an LED-filament in accordance with the invention (Dev.1) wt %photoluminescence material Front face Back face Filament KSF CASN615CASN640 YAG550 GAL535 KSF CASN640 YAG550 GAL535 Com.1 — — 7 — 93 — 7 —93 Com.2 60 — — 40 — 60 — 40 — Dev.1 56 4 — 40 — — 7 — 93

TABLE 5 tabulates the measured optical performance of the LED-filamentsCom.1, Com.2 and Dev.1. As can be seen from TABLE 5, the flux generatedby Dev.1 is 22.2 lm greater (19% brighter: Brightness—Br) thanLED-filament Com.1 that uses CASN on both front and back faces of thesubstrate. While LED-filament Com.2 generates a flux that is 33.5 lmgreater (26% brighter: Brightness—Br) than LED-filament Com.1, thisLED-filament uses double the amount of KSF (narrow-band redphotoluminescence material) as that of Dev.1. It will be appreciatedthat LED-filament Dev.1 achieves 94% (119/126) of the possiblebrightness gain of using KSF (narrow-band red photoluminescencematerial) in place of CASN, but using only half (50% by weight) theamount of KSF. This is achieved, at least in part, due to the presenceof the partially light transmissive substrate used in Dev.1. Theinvention thus discloses improvements relating to the LED-filaments andLED-filament lamps and in particular, although not exclusively, reducingthe cost of manufacture of LED-filaments without compromising onbrightness and CRI Ra.

TABLE 5 Measured optical characteristics of 0.7 W, 2700 K nominal colortemperature LED-filaments Com.1, Com.2 and Dev.1 Flux Br Light emission(%) CIE Filament (1 m) (%) Forward Backward x y CCT (K) CRI Ra Com.1115.5 100 84 16 0.4245 0.3952 3070 95.6 Com.2 145.8 126 80 20 0.43910.4175 3148 90.5 Dev.1 137.7 119 80 20 0.4821 0.4395 2624 85.0

Experimental Data—Double-Layer Structured LED-Filament

As discussed above, double-layer structured LED-filaments (FIGS. 4A and4B) compared with a single-layer structured LED-filament (FIG. 2C) canprovide a substantial reduction in usage amount of narrow-band redphotoluminescence material. Dev.2 is a single-layer LED-filament inaccordance with the invention Dev.2 and Dev.3 is a double-layerLED-filament (FIG. 4A) in accordance with the invention.

Dev.2 and Dev.3 each comprise a 38 mm by 1.5 mm porous silica substratewith a transmittance ≈40% having twenty four serially connected 714 (7mil×14 mil) blue LED chips of dominant wavelength λ_(d)=456 nm mountedon a front face. Each LED-filament is a nominal 150 lm (1 W) device andis intended to generate white light with a target Correlated ColorTemperature (CCT) of 2700K and a target general color rendering indexCRI Ra of 90. It will be appreciated that three of these LED-filamentscan be used to provide a 450 lm LED-filament lamp.

The photoluminescence materials (phosphors) used in the test devices areKSF phosphor (K₂SiF₆:Mn⁴⁺) from Intematix Corporation, CASN phosphors(Ca_(1-x)Sr_(x)AlSiN₃:Eu λ_(pe)≈615 nm, 631 nm and 640 nm), and greenYAG phosphors (Intematix GYAG4156 and GYAG543—(Y,Ba)_(3-x)(Al_(1-y)Ga_(y))₅O₁₂:Ce_(x) Peak emission wavelength λ_(pe)=543nm and 550 nm).

For the single-layer LED-filament Dev.2 the red and green phosphors weremixed in a phenyl silicone and the mixture dispensed onto respectivefront and back faces of the substrate.

For the two-layer LED-filament Dev.3, the KSF was mixed with a phenylsilicone and the mixture dispensed as a strip (first layer) onto thefront face of the substrate covering the LED chips. The green phosphorand CASN was mixed in a phenyl silicone and the mixture dispensed as asecond layer on the first layer on the front face of the substrate. Onthe back face, the green phosphor and CASN was mixed in a phenylsilicone and the mixture dispensed onto the back face of the substrate.

TABLE 6 tabulates phosphor compositions of the single-layer LED-filamentDev.2 and the double-layer LED-filament Dev.3. As can be seen from TABLE6, in terms of phosphor composition the single-layer LED-filament Dev.2comprises, on the front face of the substrate, a mixture of 74 wt % KSF,2.2 wt % CASN615 and 23.8 wt % YAG543 and on a back face of thesubstrate a mixture of 5 wt % CASN (1.4 wt % CASN631+3.6 wt % CASN650)and 95 wt % YAG550. As can be seen from TABLE 6, in terms of phosphorcomposition the double-layer LED-filament Dev.3 comprises on the frontface of the substrate a first layer comprising KSF only (17.0 wt % ofthe total phosphor content of the front face) and a second layercomprising a mixture of 7.8 wt % CASN615 and 75.2 wt % YAG543 and on aback face of the substrate a mixture of 5 wt % CASN (1.4 wt %CASN631+3.6 wt % CASN650) and 95 wt % YAG550. Dev.3 comprises on theback face of the substrate a mixture of 5 wt % CASN (1.4 wt %CASN631+3.6 wt % CASN650) and 95 wt % YAG550.

TABLE 6 Phosphor composition of a single-layer (Dev.2) and double-layer(Dev.3) LED-filaments wt % photoluminescence material Back face Frontface CASN Filament KSF CASN615 YAG543 CASN631 CASN650 YAG550 Dev.2 74.02.2 23.8 1.4 3.6 95.0 Dev.3 17.0 7.8 75.2 1.4 3.6 95.0

TABLES 7A and 7B tabulate the phosphor amounts (usage) of thesingle-layer LED-filament Dev.2 and the double-layer LED-filament Dev.3.The phosphor weight values (weight) in TABLES 7A and 7B are normalizedphosphor weight normalized to the weight of KSF of the single-layerLED-filament Dev.1.

TABLE 7A Phosphor amount of single-layer (Dev.2) and double-layer(Dev.3) LED-filaments weight-phosphor weight normalized to weight of KSFof the single-layer LED-filament Dev.1 Front face-Phosphor amount KSFCASN YAG Total CASN/(CASN + KSF) Filament weight % Weight % weight %weight % (wt %) Dev.2 1.0000 100 0.0302 100 0.3219 100 1.3521 100 2.9Dev.3 0.2044 20 0.0942 312 0.9070 282 1.2056 89 31.5

TABLE 7B Phosphor amount of single-layer (Dev.2) and double-layer(Dev.3) LED-filaments weight-phosphor weight normalized to weight of KSFof the single-layer LED-filament Dev.1 Phosphor amount Back face TotalCASN YAG Total (Front and Back) Filament weight % weight % weight %weight % Dev.2 0.0524 100 0.9968 100 1.0492 100 2.4013 100 Dev.3 0.0963184 1.8350 184 1.9313 184 3.1369 131

TABLE 8 tabulates the measured optical performance of the LED-filamentsDev.2 (single-layer) and Dev.3 (double-layer). The data are for a drivecurrent I_(F)=15 mA and drive voltage V_(F)=68.7 V and are after 3minutes of operation once the filament had reached thermal stability(Hot). As can be seen from TABLE 8, the color point of light generatedby the LED-filaments are very similar with the General CRI Ra of thedouble-layer LED-filament Dev.3 being 93.1 compared with 90.5 of thesingle-layer LED-filament Dev.2. Moreover, the flux generated by thedouble-layer LED-filament Dev.3 being 4.7 lm greater (3.0% brighter:Brightness—Br) than the flux generated by the single-layer LED-filamentDev.2. Most significantly, while the two LED-filaments generate verysimilar light emissions, as can be seen from TABLE 8, compared with thesingle-layer LED-filament Dev.2, the double-layer LED-filament Dev.3uses 80% by weight less KSF (0.2044 compared with 1.0000), as can beseen from TABLES 7A and 7B. Although the double-layer LED-filament Dev.3compared with the single-layer LED-filament Dev.2 uses more CASN (212%by weight increase on front face˜0.0942 compared with 0.0302 and 84% byweight increase on back face˜0.0963 compared with 0.0524) and YAG (182%by weight increase on front face˜0.9070 compared with 0.3219 and 84% byweight increase on back face˜1.9313 compared with 1.0492), thedouble-layer structure still provides a substantial cost saving comparedwith a single-layer structure due to huge difference in costs of CASN(about a ⅕ of the cost of KSF) and YAG (about 1/100 to 1/150 of the costof KSF) compared with KSF. It is believed that the reason for theincrease in CASN and YAG usage is that due to less blue excitation lightreaching the second phosphor layer, more CASN and YAG phosphor isrequired to generate red and green light to attain the required targetcolor.

As described above, the reduction in KSF usage is a result of locatingthe KSF in a separate layer that is in contact with (adjacent to) theLED chips. It is believed that the reason for this reduction in KSFusage amount, is that in a single-layer LED-filament Dev.2 comprising asingle photoluminescence layer comprising a mixture of KSF(manganese-activated fluoride photoluminescence material), CASN and YAG,the various photoluminescence materials have equal exposure to blueexcitation light. Since KSF has a much lower blue light absorptioncapability than YAG and CASN materials, a greater amount of KSF isnecessary to convert enough blue light to the required red emission. Bycontrast, in the double-layer LED-filament Dev.3, the KSF(manganese-activated fluoride photoluminescence material) in itsseparate respective first layer is exposed to blue excitation lightindividually without competition from the YAG and CASN; thus, more ofthe blue excitation light can be absorbed by the KSF. Since the KSF canmore effectively convert the blue excitation light to red emission, theamount (usage) of KSF (narrow-band red photoluminescence material)required to achieve a target color point can be reduced compared withLED-filaments comprising a single-layer comprising a mixture ofphotoluminescence materials.

As will be further noted from TABLE 7A, the content ratio of the CASN(broad-band red photoluminescence material) with respect to the total ofthe KSF (narrow-band red photoluminescence material) and CASN in thedouble-layer LED-filament Dev.3 is greater than about 30 wt %.

TABLE 8 Measured optical characteristics of 150 l m, 2700 KLED-filaments Dev.2 and Dev.3 Flux Br CIE CRI Ra Filament (1 m) (%) Lm/Wx y CCT (K) Ra R8 R9 Dev.2 154.5 100.0 150.0 0.4595 0.4103 2702 90.591.8 76.9 Dev.3 159.2 103.0 154.3 0.4591 0.4113 2716 93.1 87.3 67.3

Dev.4 is a further double-layer LED-filament in accordance with theinvention and is a nominal 250 lm (1.5 W) device that is intended togenerate white light with a target Correlated Color Temperature (CCT) of2700K and a target general color rendering index CRI Ra of 90. It willbe appreciated that four of these LED-filaments can be used to provide a1000 lm LED-filament lamp using for example the embodiment of FIGS. 1Aand 1B. LED-filament Dev.4 comprises a 52 mm by 3.0 mm porous silicasubstrate with a transmittance ≈40% having twenty five seriallyconnected 714 (8 mil×27 mil) blue LED chips of dominant wavelengthλ_(d)=454 nm mounted on a front face.

TABLE 9 tabulates phosphor compositions of the double-layer LED-filamentDev.4. As can be seen from TABLE 9, in terms of phosphor composition thedouble-layer LED-filament Dev.4 comprises on the front face of thesubstrate a first layer comprising KSF only (23.1 wt % of the totalphosphor content of the front face) and a second layer comprising amixture of 7.5 wt % CASN615 and 69.4 wt % YAG543. On a back face of thesubstrate a mixture of 9.1 wt % CASN615 and 90.9 wt % YAG535.

TABLE 9 Phosphor composition of a 250 lm double-layer LED-filament Dev.4wt % photoluminescence material Front face Back face Filament KSFCASN615 YAG543 CASN615 YAG535 Dev.4 23.1 7.56 9.4 9.19 0.9

TABLES 10A and 10B tabulate the phosphor amounts (mg) of thedouble-layer LED-filament Dev.4. The phosphor weight values (weight) inTABLES 10A and 10B are normalized phosphor weight normalized to theweight of KSF of a single-layer LED-filament using the samephotoluminescence materials. As can be seen from TABLE 10A adouble-layer structured LED-filament reduces KSF usage nearly 80% byweight (0.1956 compared with 1.0000) compared with a single-layerstructured LED-filament and nearly 90% by weight compared with knownLED-filaments that comprise KSF on the front and back faces. As will befurther noted from TABLE 10A, the content ratio of the CASN (broad-bandred photoluminescence material) with respect to the total of the KSF(narrow-band red photoluminescence material) and CASN in thedouble-layer LED-filament Dev.4 is about 25 wt %.

TABLE 10A Phosphor amount of 250 l m double-layer LED-filament Dev.4Front face-Phosphor amount CASN/ KSF CASN615 YAG543 Total (CASN +Filament weight % weight weight weight KSF) (wt %) Dev.4 0.1956 19.60.0642 0.7012 0.9610 24.7

TABLE 10B Phosphor amount of a double-layer LED-filament Dev.4 Phosphoramount Back face CASN615 YAG535 Total Total weight Filament weightweight weight Front & Back Dev.4 0.0493 0.3764 0.4257 1.3867

TABLE 11 tabulates the measured optical performance of the double-layerLED-filament Dev.4. The data includes measurements immediately afterswitching the filament on (referred to as Instantaneous or COLDmeasurement) and after the filament has reached thermal stability(referred to as HOT measurement) after a period of about 3 minutesoperation. Test data has shown that double-layer structured LED-filamentenable production of LED-filaments having a CRI Ra greater than 90 andan optical performance which is greater (5% to 10%) than knownLED-filament with a CRI Ra of only 80.

TABLE 11 Measured optical characteristics of a nominal 250 l m, 2700 Kdouble-layer LED-filament Dev.4 I_(F) Flux CIE CRI Ra Test Condition(mA) V_(F) (V) (1 m) Lm/W x y CCT (K) Ra R8 R9 Cold (C) 20.0 68.6 252.0183.8 0.4556 0.4148 2793 92.5 83.5 59.3 Hot (H) 20.0 67.3 234.7 174.40.4553 0.4094 2756 93.4 83.6 61.2 Δ C to H for 0.0 −1.3 93% 95% −0.0004−0.0054 −37 +0.9 +0.1 +1.9 I_(F) = 20 mA Hot (H) 22.0 67.4 254.5 171.60.4552 0.4089 2753 93.4 83.5 61.1 Hot (H) 25.0 67.6 282.9 167.3 0.45520.4083 2748 93.4 83.4 61.0

Embodiments of the invention thus concern improvements relating to theLED-filaments and LED-filament lamps and in particular, although notexclusively, reducing the cost of manufacture of LED-filaments withoutcompromising on brightness and CRI Ra.

1. (canceled)
 2. An LED-filament comprising: an at least partiallylight-transmissive substrate; an array of LED chips on a front face ofthe substrate; a first layer containing a narrow-band red phosphor,wherein the first layer is in direct contact with and covers all of theLED chips of the array; a second layer containing a first broad-bandgreen phosphor and a first broad-band red phosphor, wherein the secondlayer is in direct contact with and covers the first layer; and a thirdlayer containing a second broad-band green phosphor and a secondbroad-band red phosphor covering a back face of the substrate.
 3. TheLED-filament of claim 2, wherein the third layer contains no narrow-bandred phosphor, or wherein the third layer contains a narrow-band redphosphor in an amount up to 5 wt % of a total red phosphor content ofthe third layer.
 4. The LED-filament of claim 2, wherein a content ratioof the first broad-band red phosphor with respect to the total of thenarrow-band red phosphor and the first broad-band red phosphor is atleast one of: at least 20 wt %; at least 30 wt %; and at least 40 wt %.5. The LED-filament of claim 2, wherein the peak emission wavelength ofthe first broad-band red phosphor is shorter than at least one of: thepeak emission wavelength of the second broad-band red phosphor, and thepeak emission wavelength of the narrow-band red phosphor.
 6. TheLED-filament of claim 5, wherein the peak emission wavelength of thefirst broad-band red phosphor is about 615 nm.
 7. The LED-filament ofclaim 5, wherein the peak emission wavelength of the second broad-bandred phosphor is from about 620 nm to about 650 nm.
 8. The LED-filamentof claim 2, further comprising a layer containing particles of a lightscattering material, wherein the layer is in contact with at least oneof: the first layer, the second layer, and the third layer.
 9. TheLED-filament of claim 8, wherein light scattering material is selectedfrom the group consisting of: zinc oxide, titanium dioxide, bariumsulfate, magnesium oxide, silicon dioxide, aluminum oxide, zirconiumoxide, and mixtures thereof.
 10. The LED-filament of claim 2, whereinthe narrow-band red phosphor is at least one of: K₂SiF₆:Mn⁴⁺,K₂GeF₆:Mn⁴⁺, and K₂TiF₆:Mn⁴⁺.
 11. The LED-filament of claim 2, whereinthe substrate has a transmittance of one of: from 20% to 100%, from 2%to 70%, from 30% to 50%, and from 10% to 30%.
 12. The LED-filament ofclaim 2, wherein the LED-filament has a luminous efficacy of at least150 lm/W.
 13. The LED-filament of claim 2, wherein the first layer is onat least one light emitting face of the LED chips, or is on each lightemitting face of the LED chips.
 14. The LED-filament of claim 2, whereinat least one of the first layer, the second layer, and the third layercomprises particles of a light scattering material.
 15. The LED-filamentof claim 14, wherein light scattering material is selected from thegroup consisting of: zinc oxide, titanium dioxide, barium sulfate,magnesium oxide, silicon dioxide, aluminum oxide, zirconium oxide, andmixtures thereof.
 16. An LED-filament lamp comprising an LED-filament,wherein the LED-filament comprises: an at least partiallylight-transmissive substrate; an array of LED chips on a front face ofthe substrate; a first layer containing a narrow-band red phosphor,wherein the first layer is in direct contact with and covers all of theLED chips of the array; a second layer containing a first broad-bandgreen phosphor and a first broad-band red phosphor, wherein the secondlayer is in direct contact with and covers the first layer; and a thirdlayer on a back face of the substrate containing a second broad-bandgreen phosphor and a second broad-band red phosphor.
 17. TheLED-filament lamp of claim 16, wherein the third layer contains nonarrow-band red phosphor or wherein the third layer contains anarrow-band red phosphor in an amount up to 5 wt % of a total redphosphor content of the third layer.
 18. The LED-filament lamp of claim16, wherein the peak emission wavelength of the first broad-band redphosphor is shorter than at least one of: the peak emission wavelengthof the second broad-band red phosphor, and the peak emission wavelengthof the narrow-band red phosphor.
 19. The LED-filament lamp of claim 16,wherein the peak emission wavelength of the first broad-band redphosphor is about 615 nm and/or the peak emission wavelength of thesecond broad-band red phosphor is from about 620 nm to about 650 nm. 20.The LED-filament lamp of claim 16, further comprising a layer containingparticles of a light scattering material, wherein the layer is incontact with at least one of: the first layer, the second layer, and thethird layer.
 21. The LED-filament lamp of claim 16, wherein thenarrow-band red phosphor is at least one of: K₂SiF₆:Mn⁴⁺, K₂GeF₆:Mn⁴⁺,and K₂TiF₆:Mn⁴⁺.